FI85562C - FOERFARANDE FOER AVLAEGSNANDE AV KVAEVEOXIDER UR AVGASER, VILKA UPPKOMMER VID FOERBRAENNING. - Google Patents

Abstract

For the removal of nitrogen oxides by selective catalytic reduction with ammonia, catalysts are used which essentially contain titanium oxide and catalytically active quantities of oxides of vanadium, tungsten and/or molybdenum. The catalysts have a BET surface area from 5 to 60 m<2>/g and a pore volume of 0.15 - 0.40 ml/g and contain at most 35 % of pores of diameters < 200 A.

Description

1 85562

Method for removing nitrogen oxides from exhaust gases from coal combustion

The present invention relates to a process for the selective removal of Ν0χ from the combustion gases of coal combustion plants by reduction with ammonia.

For the removal of NOv from exhaust gases, a number of methods have previously been proposed, of which the catalytic methods for the reduction of 10 ΝΟχ: η with reducing gases have been of particular technical importance. Thus, there are methods in which ΝΟχ is reduced by means of fuel, for example natural gas or methane, with precious metals acting as catalysts non-selectively to nitrogen. These methods have not technically become relevant because, in addition to nitrogen oxides, the oxygen in the exhaust gases also reacts, resulting in higher than desired temperatures and, secondly, large amounts of fuel being required, not to mention that the precious metal catalysts are non-toxic.

For these reasons, a selective lethalytic process has also been developed in which nitrogen oxides are reduced in the presence of a catalyst of various noble metals with ammonia; wedge selectively to nitrogen. This method, known as the SCR (Selective Catalytic Reduction, Selective Catalytic:. * 25 Reduction) method, has become increasingly important.

. In this process, in practice, the oxygen in the exhaust gases does not react and the amount of ammonia is determined only by the nitric oxide content of the gases to be treated.

. ···. Various catalysts are used in this process.

U.S. Pat. No. 3,279,884 discloses catalysts containing catalytically effective amounts of vanadium, molybdenum and / or tungsten oxides. DE-PS 1 253 685 t 2 85562 also discloses catalysts which may also contain manganese oxide and / or iron oxide.

DE-PS 2 458 888 finally discloses catalysts for the same purpose which contain: 5 1. titanium as oxide and 2. at least one element from the following group a) iron and vanadium as oxides and / or sulphates and / or b) molybdenum, tungsten, nickel, cobalt, copper-10 ri, chromium and uranium as oxides and if desired 3. tin as oxide and / or 4. silver, beryllium, magnesium, zinc, boron, aluminum, yttrium, rare earth metals, silicon, niobium, antimony, bismuth and manganese The oxide.

These catalysts can be added as full catalysts or also with customary supports.

DE-OS 3,438,367 discloses catalysts having: 1. a binary oxide compound of titanium and silicon and / or titanium and zirconium and / or an oxide compound of three elements, titanium, zirconium and silicon, and 2. 0-5 by weight -% vanadium oxide and 3. 1-15% by weight of at least one oxide of tungsten, molybdenum, tin and cerium.

25 In addition to the production of nitric acid, nitrous oxide-containing exhaust gases are produced primarily in the industrial combustion of fossil fuels, for example in power stations. In addition to nitrogen oxides, : the exhaust gases from the combustion also contain sulfur dioxide, according to the composition of the fossil fuel, and the exhaust gases from the combustion of coal have, depending on the combustion method, whether or not the combustion contains dry or molten ash, 3 85562 different amounts of other components which may also damage catalysts, such as heavy metals or heavy metal compounds, halogen compounds such as HCl and HF, the amount of which increases as the combustion temperature increases, in particular heavy metal compounds such as cadmium, lead, ar compounds of selenium and selenium, poison the catalysts used, reducing their activity over time.

In summary, therefore, the catalysts in the SCR process must meet the following conditions: 1. They must be selective and / or reduce as much NO as possible while minimizing oxygen atoms.

2. they must be as inactive as possible with regard to the oxidation of SO2 in the exhaust gases to SO2. j

3. they must be as resistant as possible to the toxic effects of harmful substances in the exhaust gases I

against. 's

The above summary served as a basis for the development of a process for the removal of nitrogen oxides from exhaust gases from coal combustion by selective reduction with ammonia in the presence of catalysts containing a catalytically effective amount of vanadium, tungsten in addition to titanium oxide. . ·. 25 and / or oxides of molybdenum, a combination of active ingredients which has been shown to be relatively inactive with respect to ... Oxidation of SC 2 to Sod, whereby the added catalysts have a higher resistance to poisoning against the harmful substances present in these combustion gases.

: “30 It was now found that this problem could be solved by measuring the surface area of the catalysts 2! With BET 5-60 m / g, a pore volume of 0.15-0.40 ml / g and a porosity concentration of <200 Å in diameter, up to 35%.

<85562

Surprisingly, it is possible for these catalysts, in principle, to obtain better toxicity resistance without changing their chemical composition. This is probably due to the fact that, as our own measurements have shown, the catalysts of the invention with a relatively small surface area and lower microporosity absorb significantly less heavy metals or heavy metal compounds than catalysts with a significantly larger surface area and high microporosity. -10 in the mouth.

2

Preferably, the catalysts have a surface area of 15-50 m / g and a pore content of at most 20% in pores with a diameter of <200 Å. The catalysts according to the invention mainly contain titanium oxide and oxides of vanadium, molybdenum and tungsten, so that titanium and the catalytically active metal are present in an atomic ratio of 1: 0.0005-1. Of course, the catalysts may contain other metal oxides suitable for the present purpose, such as tin, nickel, cobalt, copper, chromium and uranium oxides.

The catalysts are prepared in the usual way by mixing the ingredients by molding, drying and calcining, as well as by precipitation, molding of the partially dried reaction mixture, further drying and calcination.

The surface area of the catalysts according to the invention is determined to be simple! for example, by adding a primary portion of the active ingredients such as titanium oxide with a particle size <200 Å or by using a certain calcination temperature and a certain calcination time. This can always vary according to the specific composition of the catalyst in question, but can be easily determined by other orientation experiments.

The catalysts can be prepared in the desired form:; for example in the form of bars, tablets, preferably; 35 into honeycomb shapes with through parallel channels. The process according to the invention is carried out at a temperature of 300-460 ° C at a specific rate of 1000-10,000 h. Ammonia is added in a molar ratio of NH 2 -NO of 0.7-1.2, preferably chosen as a molar ratio of <1, in order to prevent NH

Example 1

A suspension of metatitanic acid with 933 g of TiO2 is mixed with a solution of 169.3 g of ammonium p-tungstate in 5 l of distilled water and a solution of 5.94 g of ammonium metavanadate in 3 l of distilled water is added. . The suspension is evaporated to a plastic mass with a heated stirrer. The mixture is compressed into rods with a diameter of 5 mm, the oaks are dried (16 h, 120 ° C) and calcined at the following temperatures for 3 hours.

15 1) 500 ° C (Catalyst A) 2) 600 ° C (Catalyst A 1) 3) 700 ° C (Catalyst A 2) and 4) 800 ° C (Catalyst A3) t

The Ti: W: V atomic ratio of the obtained catalysts was 94.36: 5.24: 0.4. !

Example 2

As shown in Example 1, Catalysts A1-A3 were prepared with the following Ti: W: V atomic ratios, respectively, varying the amount of ammonium metavanadate added: 1) Catalyst B1 with a Ti: W: V atomic ratio of 94.27: 5, 24: 0.49. (Calcination temperature as for catalyst A 1) ... 2) Catalyst B 2 with a Ti-W: V atomic ratio of 94.14: •; 5.23: 0.63. (Calcination temperature such as catalyst A 2): 3) Catalyst B 3 with a Ti: W: V atomic ratio of 92.21: 5.12:: · 30 2.67. (Calcination temperature such as catalyst A 3)

Example 3

Catalysts A-A3 and B1-B3 prepared according to Examples 1-2 were subjected to experimental tests with a synthetic flue gas having the following composition: . 35 1% v / v NO, 1% v / v NHy 0.3% v / v SO2, 5% v / v [02, 10% v / v water vapor, 0.0037% v / v As ^ Og (with [: i, t <• r 6 85562 were stimulated with heavy metal contaminants in carbon flue gases), the rest with nitrogen /, at a temperature of 330 ° C and a specific rate of 93,000 h 1.

In each case, 1 g of catalyst reduced to a particle size of 0.5-1 mm was added. The diameter of the experimental reactor was 6 mm

The following table gives the NOx removal and arsenic content obtained with the individual catalysts as well as the BET surface area, pore concentration <10A smaller diameter and pore volume.

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The table above shows the initial activity of the reference catalyst A = 1 and the catalytic activities of the others compared to this. In this case, a velocity constant was used, which was determined by the following equation: 5 K = - SV x 1n (1 - η N0V) λ where SV is the specific velocity (93,000) and η ΝΟχ is the measure ΝΟχ change.

When reporting a decrease in activity, the initial activity of each catalyst is divided by the activity obtained after the 6 hour test period.

It can be seen from the table that the catalysts in the set A-A3 each have a similar vanadium content but different BET areas, <2000A pore concentration and pore volumes and the resulting 15 different initial activities and a smaller decrease in activity and a lower As content at the end of the test period then when they have the parameters according to the invention (Catalysts A1-A3) compared to the reference catalyst A.

The lower relative initial activity of the catalysts A1-A3 according to the invention can be eliminated by increasing the vanadium content, as can be seen by comparing the results with catalysts B1-B3, without affecting the decrease in activity and the As content after 6 hours *.

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Claims (3)

9 85562 1. A process for the removal of nitrogen oxides from exhaust gases produced by the combustion of coal, selectively by catalytic reduction with ammonia in the presence of catalysts containing mainly titanium oxide and catalytic amounts of oxides of vanadium, tungsten and / or molybdenum, characterized in that the catalysts The area measured in BET is 5-60 m / g, the pore volume is 0.15-0.40 ml / g and the pore content of the pores <200A is at most 35%. Method according to Claim 1, characterized in that the content of pores with a diameter of less than 15,200A is at most 20%. Process according to Claims 1 and 2, characterized in that the surface area is 10 to 50 m 2 / g. I S? • · · • M · • «• · • · I1> • · · • · 1» • · • · · · • «· • · * 1 1 • • • · · ♦ · 1 m · · Ίο 8 5562